Method and apparatus for vacuum casting of metal

ABSTRACT

A method and apparatus for vacuum casting metal includes a furnace for forming a bath of molten metal in situ. A vertical sprue has its lower end immersed in the bath of molten metal in the furnace, with its upper end extending above the surface level of the bath and into a vacuum chamber. Within the vacuum chamber, a metal stripper plate having an aperture in fluid communication with the top of the sprue forms a fixed casting station. A plurality of vacuum casting molds are conveyed in seriatim into register with the casting station within the vacuum chamber, with an entry port in the bottom of each mold being brought into fluid communication with the aperture in the stripper plate. Each time a mold enters the vacuum chamber, the chamber is sealed and evacuated to draw molten metal up through the sprue and the stripper plate into the mold. Downward pressure is applied to the mold during the casting operation to prevent the leakage of molten metal from the path of fluid communication between the mold and the stripper plate. After the cast metal has solidified in the entry port of the mold and in the stripper plate, the solidified metal is broken between the mold and the stripper plate and any solidified metal remaining in the stripper plate and the sprue is ejected into the bath of molten metal at the lower end of the sprue. The vacuum chamber is then opened, the full mold is removed therefrom, and a new mold is transferred into the vacuum chamber.

Weiler et a1.

[451 Jan. 28, 1975 METHOD AND APPARATUS FOR VACUUM CASTING 0F METAL Donald ,1. Weiler; John H. Fritz; B. Frank Lewis, all of Aurora, 111.

Aurora Metal Corporation, Aurora, 111.

Filed: Feb. 16, 1973 Appl. No.: 333,070

[75] Inventors:

[73] Assignee:

References Cited UNITED STATES PATENTS 1,473,246 11/1923 Montupet..... 164/256 FOREIGN PATENTS OR APPLICATIONS 233,362 11/1925 Great Britain 164/306 477,384 7/1915 France 164/309 11,190 12/1913 Great Britain 164/309 Primary ExaminerFraneis S. Husar Assistant Examiner-V. K. Rising Attorney, Agent, or FirmWolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[57] ABSTRACT A method and apparatus for vacuum casting metal ineludes a furnace for forming a bath of molten metal in situ. A vertical sprue has its lower end immersed in the bath of molten metal in the furnace, with its upper end extending above the surface level of the bath and into a vacuum chamber. Within the vacuum chamber, a metal stripper plate having an aperture in fluid communication with the top of the sprue forms a fixed casting station. A plurality of vacuumv casting molds are conveyed in seriatim into register with the casting station within the vacuum chamber, with an entry port in the bottom of each mold being brought into fluid communication with the aperture in the stripper plate. Each time a mold enters the vacuum chamber, the chamber is sealed and evacuated to draw molten metal up through the sprue and the stripper plate into the mold. Downwardpressurc is applied to the mold during the casting operation to prevent the leakage of molten metal from the path of fluid communication between the mold and the stripper plate. After the cast metal'has solidified in the entry port of the mold and in the stripper plate, the solidified metal is broken between the mold and the stripper plate and any solidified metal remaining in the stripper plate and the sprue is ejected into the bath of molten metal at the lower end of the sprue. The vacuum chamber is then opened, the full mold is removed therefrom, and a new mold is transferred into the vacuum chamber.

METHOD AND APPARATUS FOR VACUUM CASTING OF METAL DESCRIPTION OF THE INVENTION The present invention relates generally to metal casting and, more particularly, to a method and apparatus for vacuum casting metal.

It is a primary object of the present invention to provide an improved method and apparatus for vacuum casting metal at high production rates and low production costs from both the labor and material standpoint.

It is another object of the invention to provide an improved vacuum casting method and apparatus of the foregoing type which provides increased yield of the metal being cast. In this connection, a related object of the invention is to provide such a method and apparatus which reduces the quantity of metal that must be cut off the cast articles and remelted.

A further object of the invention is to provide-an improved vacuum casting method and apparatus of the type described above which is applicable to the casting of a wide variety of different metals, including both ferrous and non-ferrous metals.

Still another object of the invention'is to provide such an improved vacuum casting method and apparatus which permits the casting of articles of varying sizes and shapes one after the other and in any desired order on a single production line. A related object is to provide such a method and apparatus which can be used to cast any desired number of articles of any given configuration.

It is a still further object of the invention to provide such an improved vacuum casting method and apparatus which is capable of use with a variety of different casting designs involving casting molds utilizing steel cores, multiple mold sections, inserts, disintegratable cores and the like. I

Yet another object of the invention is to provide such an improved vacuum casting method and apparatus which reduces fuel cost due to lower operating temperatures and reduced heat losses. In this connection, corollary objects of the invention are to provide a longer mold life due to reduced wear on the mold at the lower operating temperatures, and improved working conditions because of the lower ambient temperatures made possible by the invention.

A further object of the invention is to provide such an improved vacuum casting method and apparatus which improves working conditions by reducing the noise level, and by providing an apparatus which is completely enclosed so that safety hazards, emissions and odors are negligible.

Another object-of the invention is to provide such an improved vacuum casting apparatus which has a long operating life.

Other objects and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of a vacuum casting machine embodying the present invention;

FIG. 2 is an enlarged side elevation taken along line 2-2 in FIG. 1, with the vacuum chamber closed;

FIG. 3 is the same side elevation shown in FIG. 2 but with the vacuum chamber open, and showing a mold entering the vacuum chamber;

FiG. 4 is a vertical section taken through the middle of the apparatus shown in FIG. 3, along a line parallel to the direction of mold movement, and showing a mold in position for a casting cycle within the vacuum 5 chamber, with the mold and a portion of the furnace beneath the vacuum chamber shown in full elevation;

FIG. 5 is an enlarged plan view taken along line 55 in FIG. 4;

FIG. 6 is an enlarged section taken along line 66 in FIG. 4, with the mold support and clamping mechanism in its raised position;

FIG. 7 is the same section shown in FIG. 6 but with the mold support and clamping mechanism in its lowered position, and showing a greater portion of the furnace beneath the vacuum chamber; and

FIG. 8 is an enlarged side elevation of the cam which operates pivoted stop members on each mold just before it enters the vacuum chamber, and illustrating the different positions of an exemplary stop member on op posite sides of the cam.

While the invention will be described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of the invention.

Turning now to the drawings and referring first to FIG. 1, there is shown a vacuum casting machine for casting articles for molten metal contained in a conventional furnace 10. The furnace l0 typically includes a refractory container 11 for holding the metal to be 7 melted and the resulting bath of molten metal 12 (FIG.

7), and may be surrounded by thermal insulation (not shown) if desired. The metal within the refractory container 11 may be melted by any suitable means, by induction heating is preferred because of its quietness. For example, the furnace may be a VIP 165 KW induction heating furnace made by Inductotherm Corporation, Rancocas, N]. r

In accordance with one important aspect of the invention, a series of vacuum casting molds are conveyed in seriatim through a vacuum chamber located above the bath of molten metal in the furnace 10, and each mold dwells with the vacuum chamber for a period sufficient to permit the mold to be filled with molten metal. Thus, in the illustrative machine a vacuum chamber 13 formed by a housing 14 mounted directly over the furnace has a pair of doors 15 and 16 on opposite sides thereof for cooperation with two sections 17 and 18 of a track located on opposite sides of the housing 14 for conveying a series of molds 19 to and from the housing 14. The doors l5 and 16 are periodically opened to permit ingress and egress of the molds 19 to and from the vacuum chamber 13, and then closed to seal the vacuum chamber to permit the casting of metal from the furnace 10 into the particular mold located in the vacuum chamber. Each time a casting operaion is completed, the doors l5 and 16 are opened, the mold 19 in the vacuum chamber 13 is transferred onto the tracks 18, a new mold 19 enters the vacuum chamber via tracks 17, and the doors are then closed again to seal the chamber for another casting operation. This cyclic operation is iterated at a rapid rate so that a relatively large number of castings can be formed in any given time period, and with very little manual labor.

The casting system provided by this invention is particularly suitable for automatic or semi-automatic recycling of the molds 1 9 between the exit and entrance side of the vacuum chamber 13. Thus, the molds may be conveyed from the exit side of the casting station along the tracks 18 to a stripping station where the castings are removed from the molds, after which the molds are immediately returned to the casting station along the tracks 17. In this type of operation, it is preferred that the tracks 17 and 18 form a closed loop. and the tracks may be inclined to convey the molds around the loops by gravity with a single elevator located at some point in the loop, suitably at the stripping station, to raise the molds from the low end to the high end of the closed-loop formed by the tracks 17 and 18. Thus, it can be seen that the overall operation is not only rapid, but also extremely efficient, requiring only a minimum amount of manual labor.

Returning now to a more detailed description of the casting station, the opening of the entrance door causes track members on the outer surface of the door to form a continuation of the track section 17 for transferring the molds 19 onto similar track members 21 within the vacuum chamber 13 (see FIG. 3). Similarly, when the exit-door 16 is open, track members 22 on the outer surface thereof form a continuation of the track section 18 for transferring molds 19 from the internal track 21 onto the section 18. Thus, it can be seen that the combination of the two track sections 17 and 18, the tracks 20 and 22 formed on the two doors, and the internal track 21 form a continuous track for conveying molds through the vacuum chamber 13.

To carry the molds 19 over the track assembly described above, each mold is placed on a mold carriage (FIGS. 3-8) comprinsing a bed plate 31 forming a large center opening 31a and having four wheels 32-35 journalled in the four corners of the plate for riding on the parallel tracks 17, 18, 20, 21 and 22. Carried on top of the bed plate 31 is a chill plate 36 forming a raised central land 37 adapted to register with a complementary recess formed in the bottom of the mold 19 to prevent any horizontal movement of the mold while it is on the carriage. The mold 19 rest on top of the chill plate 36, and the center of the plate 31 forms a vertical aperture 38 extending therethrough to provide access to the mold 19 from beneath the carriage bed plate 36.

In order to open the doors 15 and 16 of the vacuum chamber to permit ingress and egress of the mold carriages 30, a pair of hydraulic cylinders and 51 pivot the doors about a pair of fixed pivot points 52 and 53. As indicated by the arrow adjacent the entrance door 15 in FIG. 2, the doors 1S and 16 are linked to the respective pivot points 52 and 53 in such a way that the bottom of the doors swing outwardly toward the respective adjacent track sections 17 and 18 while the tops of the doors slide downwardly over the ends of the housing 14. The purpose of this particular patttern is pivotal movement, of course, is to position the tracks 20 and 22 on the top surfaces of the respective doors 15 and 16 when they are in their horizontal, open positions, thereby bridging the spaces between the internal track sections 21 and the respective external track sections 17 and'l8. In FIG. 2, the doors are completely closed; in FIG. 3 they are completly open.

As the cylinder 50 and 51 are advanced, they drive a pair of corresponding rods 54 and 55 downwardly, thereby lowering the upper ends of two pairs of links 56 and 57 pivotally connected to two brackets 58 and 59 carried on the bottom ends of the rods 54 and 55 respectively. As the upper ends of the links 56 and 57 are lowered, they rotate two corresponding pairs of gears 60 and 61 connected to the lower ends of the links. and the gears 60 and 61 in turn to rotate two cooperating pairs of gears 62 and 63. The shafts of the latter gears 62 and 63 form the fixed pivot points 52 and 53 mentioned previously, and are connected to the respective doors 15 and 16 via links 64 and 65. When the gears 62 and 63 are rotated, they pivot the links 64 and 65 down and away from the respective ends of the housing 14, thereby moving the doors 15 and 16 from their closed positions (FIG. 2) to their open positions (FIG. 3) and bringing the tracks 20 and 22 on the outside surfaces of the doors into line with the respective external track sections 17 and 18 and the internal track section 21. The support the doors 15 and 16in their open positions, two additional pairs of links 66 and 67 are pivoted at their upper ends to the respective doors and at their lower ends to the machine frame, thereby forming supporting braces for the doors as they opened and while they are in their open positions.

As shown most clearly in FIG. 2 and 3, the distance between the brackets 58 and 59 and the corresponding gears 60 and 61 becomes progressively shorter as the rods 54 and 55 are lowered. To allow for this shortening, each of the links 56.and 57 forms an elongated slot 56a or 57a extending downwardly, from the points at which they are connected to the respective brackets 58 and 59 so that the links 56 and 57 are free to slide over each other as they are lowered.

As each carriage 30 approaches the open vacuum chamber 13, a pair of depending stops 40 mounted on the trailing end of the carriage bed plate 31 are automatically lowered to their advanced operative positions by a pair of cams 41 mounted near the end of the track section 17. More specifically, each stop 40 is pivotally fastened at one end to the bed plate 31 by a shoulder screw 42, while the other end of the stop carries a laterally projecting cam follower pin 43. When the carriage approaches the cams 41, the stop 40 is in its retracted inoperative position (right hand end of FIG. 8), the limit of which is. controlled by a spring-loaded plunger 44 riding in an arcuate slot 45 in the side wall of the plate 31. As the carriage traverses the earns 41, the pins 43 engage ramps 46 which cam the pins 43 downwardly, thereby pivoting the stops 40 to their advanced operative positions (left-hand end of FIG. 8), the limit of which is again controlled by the plunger 44 riding in the slot 45.

As the carriage 30 enters the open vacuum chamber 13, a pair of flat-faced pins 47 projecting laterally from the stops 40 engage the entrance side of a bridge assembly which normally supports the internal track members 21 slightly above the bottom wall 48 of the housing 14. As can be seen most clearly in FIGS. 4-7, the base of the bridge assembly comprises two pairs of transverse channel members 70 and 71 connected to a pair of longitudinal base members 72 and 73. To form a central opening in the base assembly for providing access to the molds19 from beneath the bridge assembly, the two pairs of transverse channel members 70 and 71 are spaced apart from each other in the longitudinal direction, and the two longitudinal members 72 and 73 are spaced apart from each other in the transverse direction. The carriage shop pins 47 engage the ends of the longitudinal base members 72 and 73, and are posi-- tioned to register the central opening 310 in the carriage bed plate 31 with the central opening in the bridge base to provide access to the bottom of the chill plate 36 on the carriage 30.

After the mold carriage 30 enters the housing 14 and is stopped against the base of the bridge assembly therein, the doors and 16 are closed by simply reversing the operation described above for opening the doors. That is, the cylinder rods 54 and 55 are raised from the positions shown in FIG. 3 to the positions shown in FIG. 2, thereby returning the doors l5 and 16 to their closed positions. The doors 15 and 16 are both provided with sealing gaskets (not shown) for engaging the end faces of the housing 14 when the doors are closed, thereby forming a tight seal between the housing and the doors to facilitate evacuation of the housing 14. To evacuate the housing, a vacuum is drawn on a line 74 connected to an aperture in the top wall of the housing.

Before the housing 14 is evacuated, the mold carriage 30 is lowered to bring the bottom of the chill plate 36 into engagement with the top of an apertured stripper plate 80 fastened to the top ofa sprue holder 81 extends up through the central opening in-the base of the bridge assembly. This sprue holder 81 is supported on the bottom of the housing 14 by a peripheral flange 81a and extends downwardly through the bottom wall of the housing 14 to form a vertical passageway which is threaded to receive the upper end of a sprue 82 extend ing down into the bath of molten metal in the furnace 11. The sprue 82 is made of a suitable refractory material such as graphite. To prevent the leakage of air through the bottom of the housing 14, a seal 83 is fitted over the flange 81a of the sprue holder 81 and bonded to both the holder 81 and the bottom of the housing 14.

Thus. it can be seen that a continuous passageway is provided between the mold 19 and the molten metal bath 12 by the sprue 82, the sprue holder 8|, the stripper plate 80, and the chill plate 36 so that molten metal can be drawn upwardly from the bath 12 into the mold 19. The headspace of the furnace 11 is at atmospheric pressure so that when the chamber 13 is evacuated, a pressure differential is produced which causes the mo]- ten metal to flow upwardly through the sprue 18 and into the mold 19. It will be understood, of course, that the mold 19 is provided with suitable vents to permit the interior cavity of the mold to be evacuated without permitting molten metal to escape throught the vents, as is customary in vacuum casting molds.

For lowering the mold carriage 30 relative to the sprue holder 81, opposite sides of the base of the bridge assembly inside the vacuum chamber 13 are connected to a pair of vertical bars 84 and 85 which extend upwardly therefrom and are connected at their upper ends to a top crossbar 86. The central portion of the crossbar 86 rests on the top of an enlarged head 87 of a piston 88 driven by a hydraulic cylinder 89. Thus, the entire bridge assembly is suspended from the piston 88 so that when the piston 89 is lowered, the bridge assembly descends along with the piston until the base of the bridge assembly engages the floor of the housing 14. Before this point is reached, however, the bed plate 31 of the mold carriage 30 is telescoped down over the stripper plate 80 so that the bottom .of the chill plate 36 is brought into engagement with the top of the stripper plate 80, as shown in FIG. 7. Continued lowering movement of the bridge assembly then disengages the carriage 30 from the bridge base, including the tracks 21, on the top of the stripper plate 80.

In keeping with the invention, the mold 19 and the chill plate 36 are pressed firmly down against the top of the stripper plate to prevent the leakage of any molten metal along the interface of these members. Thus, in the illustrative machine downward movement of the hydraulic piston 88 is continued after engagement of the bridge base with the floor of the housing 14 until the piston head 87 engages the top of the mold 19. In this position, which is illustrated in FIG. 7, the hydraulic pressure on the piston 88 urges the mold 19. the chill plate 36 and the stripper plate 80 into tight engagement with each other, thereby forming a tight passageway through which the molten metal can be safely cast into the mold. The actual casting step is then carried out by simply evacuating the chamber 13 via vacuum line 74. g

The lowering of the bridge assembly also automatically resets the carriage stops 40 by pivoting them to their inoperative retracted or raised positions. Thus, as the bridge lowers the carriage 30, the depending arms of the stops carrying the pins 43 engage a pair of pads mounted on the floor of the housing 14, thereby pivoting the stops 40 upwardly about the pivot points formed by the screws 42, i.e., from the left-hand position in FIG. 8 to the right-hand position. This releases the stop pins 47 from the bridge assembly, but the carriage 30 is still held in position by the registration of the stripper plate 80 with the central opening in the carriage bed plate 31 and the recess in the bottom of the chill plate 36 (see FIG. 7). Then when the bridge assembly is raised to disengage the carriage 30 from the stripper plate 30, and to re-engage the carriage wheels with the tracks 21, the carriage is free to be transferred out of the vacuum chamber.

After the casting step is completed by filling the mold 19 with molten metal, the vacuum drawn on the line 74 is terminated, and the molten metal is allowed to solidify in the chill plate 36, in the mold entry aperture of gate above the chill plate, and in the stripper plate 80 of the'sprue beneath the chill plate. Then the hydraulic piston 84 is retracted to remove the downward pressure from the mold 19 and to re-engage the tracks 21 with the wheels of the carriage 30, and then to return the bridge assembly and the mold carriage 30 to their starting positions shown in FIG. 6.

In accordance with one important aspect of the invention, the diameter of the aperture in the stripper plate 80 is smaller at the top then at the bottom so that upward movement of the mold away from the stripper plate breaks the metal solidified between the stripper plate and the mold at the upper surface of the stripper plate, while retaining any metal solidified in the stripper plate below the upper surface thereof. Thus, in the illustrative arrangement the aperture in the stripper plate is tapered outwardly and downwardly from the top thereof so that as the chill plate 36 is raised off the stripper plate 80, the stem of solidified metal therein is broken at the interface between the chill plate 36 and the stripper plate 80, and any solidified metal remaining in the stripper plate 80 and sprue 82 drops back into the furnace bath 12. The plug of solid metal remaining in the chill plate 36 and the mold gate prevents the escape of any metal that is still in the molten state within the mold cavity.

To permit the mold carriage 30 to be transferred out of the vacuum chamber and to permit a new carriage to enter, the doors l and 16 of the vacuum chamber are opened again in the same manner described previously. The carriage containing the full mold is then transferred from the housing 14 onto the track section 18, which may be accomplished manually or by gravity, and the casting station is ready to receive another mold carriage.

In accordance with one particular aspect of this invention, the bath of molten metal that is formed in situ in the furnace 11 is maintained in an atmosphere which is an atmosphere which is inert to the molten metal. For example, nitrogen gas is preferred inert atmosphere for most metals. Thus, in the illustrative arrangement the surface of the molten metal bath 12 in the furnace 11 is continuously blanketed with nitrogen, or any other suitable inert gas, supplied under a slight pressure through a tube 91 (FIGS. 6 and '7). This tube 91 is positioned on the opposite side of the furnace headspace from a charging port 92 through which the furnace 11 is charged with the metal to be melted, so that any nitrogen that escapes from the furnace headspace through the charging port must first flow across the surface of the molten metal. It should also be noted that this arrangement requires any metal that is charged into the furnace to pass through the nitrogen atmo-- sphere, so the charging of the furnace does not cause the molten metal to be exposed to the ambient atmosphere existing outside the furnace. Moreover, the sprue 82 extends well below the surface of the metal bath so that any oxidized metal that might be present at the bath surface is never cast into a mold in the vacuum chamber 13. A slidable cover 93 closes the charging port 92 when metal is not being charged to retain the inert gas supplied via tube 91 in the furnace headspace.

As can be seen from the tion the method and apparatus provided by this invention permit the production of high quality castings at high production rates and low production costs since only a minimum amount of manual labor is required. The process also reduces material costs by increasing the metal yield, both by reducing the quantity of metal that must be cutt off the cast articles and remelted, and by reducing the quantity of metal that is oxidized at the surface of the furnace bath. It has been found that the casting system provided by this invention is applicable to a wide variety of different metals, both ferrous and non-ferrous, and that it is capable of use with a variety of different casting designs involving casting molds utilizing steel cores, multiple mold sections, inserts, disintegratable cores and the like. Moreover, the casting apparatus is universally applicable to virtually any type of mold that can be situated on the mold carriages, thereby permitting the casting of articles of varying sizes and shapes in series with each other and in any desired order on a single production line. If desired, the apparatus can be used to cast only one item of any particular design, or any desired number greater than one, without any significant change in the production time required. Furthermore, by casting the metal directly foregoing detailed descripfrom the furnace in which it is melted into the molds, the casting system of this invention reduces both fuel costs and heat losses and permits operation at lower temperatures. The lower temperatures, in turn, provide longer mold life and improved working conditions by reducing the ambient temperature. Also, the apparatus is relatively quiet and almost completely enclosed, so that safety hazards, emissions and odors are negligible. Finally, the apparatus has a long operating life.

We claim:

1. A method of vacuum casting metal comprising the steps of: I

a. forming a bath of molten metal;

b. immersing the lower end of a vertical sprue in said bath with the upper end of the sprue extending above the surface level of the bath;

c. providing a metal stripper plate having an aperture extending therethrough in fluid communication with the top of said sprue;

d. bringing a vacuum casting mold having an enlarged chill plate on the bottom thereof into fluid communication with said stripper plate;

e. pressing said mold toward said stripper plate to prevent the leakage of molten metal from the path of fluid communication between said mold, chill plate and stripper plate;

f. forming a .vacuum chamber around said mold and withdrawing air therefrom to draw molten metal upwardly from said bath through said sprue, stripper plate and chill plate into said mold until said mold is filled;

g. solidifying the metal in the path of fluid communication through said stripper plate, chill plate and mold;

h. releasing the pressure of said mold and breaking the solidified metal between said chill plate and said stripper plate by vertically separating said mold and chill plate from said stripper plate;

. opening said vacuum chamber and removing said mold therefrom;

j. and ejecting any solidified metal remaining in said stripper plate and sprue into said bath of molten metal.

2. A method as set forth in claim 1 wherein the surface of said bath of molten metal is maintained in an atmosphere which is inert to said molten metal.

3. A method as set forth in claim 1 wherein steps (d) through (j) are cyclically repeated with a plurality of molds advanced in seriatim into register with said stripper plate, and said bath of molten metal is replenished as metal is withdrawn therefrom to fill said molds.

4. A method as set forth in claim 3 wherein said bath of molten metal is maintained in an atmosphere which is inert to said molten metal, and said bath is replenished with metal passed through said inert atmosphere.

5. A method as set forth in claim 1 wherein each of said molds and chill plates is carried on a carriage, and the vertical separation of said mold and chill plate from said stripper plate is effected by raising tracks for said carriage within said vacuum chamber. 

1. A method of vacuum casting metal comprising the steps of: a. forming a bath of molten metal; b. immersing the lower end of a vertical sprue in said bath with the upper end of the sprue extending above the surface level of the bath; c. providing a metal stripper plate having an aperture extending therethrough in fluid communication with the top of said sprue; d. bringing a vacuum casting mold having an enlarged chill plate on the bottom thereof into fluid communication with said stripper plate; e. pressing said mold toward said stripper plate to prevent the leakage of molten metal from the path of fluid communication between said mold, chill plate and stripper plate; f. forming a vacuum chamber around said mold and withdrawing air therefrom to draw molten metal upwardly from said bath through said sprue, stripper plate and chill plate into said mold until said mold is filled; g. solidifying the metal in the path of fluid communication through said stripper plate, chill plate and mold; h. releasing the pressure of said mold and breaking the solidified metal between said chill plate and said stripper plate by vertically separating said mold and chill plate from said stripper plate; i. opening said vacuum chamber and removing said mold therefrom; j. and ejecting any solidifiEd metal remaining in said stripper plate and sprue into said bath of molten metal.
 2. A method as set forth in claim 1 wherein the surface of said bath of molten metal is maintained in an atmosphere which is inert to said molten metal.
 3. A method as set forth in claim 1 wherein steps (d) through (j) are cyclically repeated with a plurality of molds advanced in seriatim into register with said stripper plate, and said bath of molten metal is replenished as metal is withdrawn therefrom to fill said molds.
 4. A method as set forth in claim 3 wherein said bath of molten metal is maintained in an atmosphere which is inert to said molten metal, and said bath is replenished with metal passed through said inert atmosphere.
 5. A method as set forth in claim 1 wherein each of said molds and chill plates is carried on a carriage, and the vertical separation of said mold and chill plate from said stripper plate is effected by raising tracks for said carriage within said vacuum chamber. 